I don't think so. UnsafeAtomic just has a single stored property which is a pointer to the atomic representation.
The newly proposed Atomics in the standard library also only have the memory footprint of the underlying atomic value representation and are even stored inline. Note that Atomics shouldn't be stored tightly packed because synchronisation usually happens at the granularity of a cache line. AFAIC there isn't currently a way to add padding automatically but you can do it manually.